[meteorite-list] Watching Comet Tempel 1

[Ron Baalke]

http://skyandtelescope.com/observing/highlights/article_1522_1.asp

Watching Comet Tempel 1
By Greg Bryant and Alan M. MacRobert
Sky & Telescope
June 6, 2005

Even if this were an ordinary year, the periodic comet 9P/Tempel 1 would
be an inviting target for medium and large amateur telescopes this
spring and summer (Northern Hemisphere seasons). The comet returns every
5.5 years. Normally it would be expected to remain as dim as 10th or
11th magnitude for all of this year's apparition - but NASA's Deep
Impact probe (the cover story of the June Sky & Telescope) is on target
to smash into the comet's icy nucleus on July 4th Universal Time (the
evening of July 3rd for North America). The resulting debris cloud, in
addition to providing a better understanding of cometary interiors, may
brighten Tempel 1 dramatically.

Amateurs worldwide are being urged to help in the efforts to study the
comet before, during, and after the impact (for details of this compaign
see page 70 of the June issue; an 8-inch telescope and a CCD camera are
required). But lots more observers will be watching just to see what
happens, and to be a part of this historic event.

Tempel 1 is already very well placed right after dark, though it's not
at all bright. If your telescope will show the brighter NGC galaxies, it
should show this comet.

Comet chart through June 23
<javascript:openLargeImage('/mm_images/8244.gif', 550, 628);>
Use this finder chart with your telescope to locate Comet Tempel 1 as
described in the text. It's glowing dimly at 10th magnitude not far from
Jupiter and Gamma Virginis. New charts will be posted when this one runs
out - or see the June Sky & Telescope, page 68. Click for full size
view. Sky & Telescope diagram.

 
 

Use the map here to locate the comet's exact position on any night from
now through June 22nd. (Click on the chart for the full-size version,
the one to print and take to the telescope.) Comet symbols are shown at
0:00 Universal Time on the dates indicated; in the Americas, this is on
the evening of the previous date. The tails on the symbols point in the
direction away from the Sun. Stars are plotted to magnitude 8.5, about
as faint as a high-quality finderscope will show. Galaxies (red ovals)
are plotted to visual magnitude 11.5 near the comet's path and 10.0
elsewhere.

The scale of the map, in degrees, is indicated by the declination ticks
5° apart along the right-hand edge. To use the map, you'll need to judge
from this scale how much of the map will appear in your finderscope's
view at a time. Good finderscopes typically have a field roughly 5° wide.

North is upper left on the map; be sure to match this direction to
celestial north in your telescope or finderscope. To do this, nudge your
telescope slightly in the direction of Polaris while looking through the
eyepiece, and note the edge from which new stars enter the field. This
is the north edge. Turn the map around so north on the map matches this
direction.

Note: if your finderscope has a right-angle bend at the eyepiece, it
probably shows you a mirror image, which will not match the map. In this
case, you can try to flip the star patterns on the map left-for-right in
your head before comparing them with what you see. Or use one of the
other solutions to the mirror-image problem described in our article
"Using a Map at the Telescope".
<http://skyandtelescope.com/howto/scopes/article_148_1.asp>

In early June, Tempel 1 passes within 8° northeast of Jupiter. Around
the middle of June moonlight will interfere with the view, but late in
the month the Moon is again out of the evening sky and Tempel 1 will be
within 3° of Theta Virginis, magnitude 4.4.

Sky & Telescope editors Joshua Roth and Tony Flanders observed Tempel 1
on the evening of June 3rd under dark but hazy skies using a 12.5-inch
reflector at 60 power. "An elongated blur shaped like a watermelon seed,
no bright nucleus" described Roth. Flanders called the comet quite
large, maybe 15 arcminutes across, but with a very low surface
brightness that made it hard to see through the imperfect sky even with
such a big telescope. Under crystal-clear dark skies, by contrast, some
observers have reported finding Tempel 1 with a 3-inch scope.

Impact Day

The Deep Impact probe's copper projectile is scheduled to slam into the
comet's nucleus around 6:00 Universal Time July 4th, which is 11 p.m.
Pacific Daylight Time July 3rd. This time was chosen so that at least
two Deep Space Network tracking stations can be receiving live data from
the probe during the action. The timing favors mostly the Pacific Ocean,
as shown on the world map below; this is the side of the world facing
the comet at impact time. (In all other parts of the world, the comet
will be below the horizon.)

Comet-facing hemisphere
This is the hemisphere of Earth that will be facing the comet at impact
time. It will be broad daylight in Japan, China, and Australia, twilight
in New Zealand and Hawaii, and dark night in Mexico and the western
United States.  Sky & Telescope diagram.

 
 

Observers in the far west of North America get lucky. Tempel 1 (just
3.5° northeast of Spica) will be in the southwestern sky, though getting
rather low. The farther southwest you are the better. The comet will be
in a fully dark sky 25° above the horizon for Los Angeles and San
Francisco, and 22° up for Tucson. There will be no Moon. Seattle and
Hawaii will be in evening twilight; the comet may be invisible from
there during impact, depending on the exact timing.

Evening twilight will be bright for observers in New Zealand, where the
local date is July 4th. Eastern Australia won't have a dark sky until at
least two hours later.

What will we see? Comet specialists predict that the nucleus may
brighten to 6th magnitude as the debris cloud slowly expands. How long
this brightness will last is uncertain. Comets that flare up as a result
of splitting apart can remain anomalously bright for months or years.
When comet 73P/Schwassmann-Wachmann 3 split in 1995, for example, it
became 7 magnitudes brighter than before, and when it returned to
perihelion six years later, it was still several magnitudes brighter
than before it fragmented. On the other hand, outbursts from comets that
remain intact (as Tempel 1 is expected to do) tend to last only days or
weeks. But weather permitting, observers all around the globe will get
good looks at Tempel 1 within 24 hours.

After Impact

In the following weeks the comet will continue its southeastward trek
through Virgo into Libra and Scorpius, passing Antares in early
September. With its increasingly southern declination and its shrinking
elongation from the Sun in our sky (73° by October 15th), observers in
the Southern Hemisphere will be favored during this time. A chart of the
comet's path through August and the first half of September is in the
June Sky & Telescope, page 69.

When the fading comet finally disappears into the glare of the Sun late
in the year, it will mark not only the end of a very special apparition
but also the end of an era for this comet. Approaches to Jupiter in 2024
and 2036 will move Tempel 1's perihelion distance from its current 1.5
a.u. to 2.0 a.u. Not until June 2122, when the perihelion distance
shortens to 1.6 a.u., is the comet expected to again appear brighter
than 11th magnitude.